Alfalfa Insects

Beet Armyworm, Spodoptera exigua.

Small cluster of beet armyworm eggs. Note covering of anal hairs.

Hatching cluster of beet armyworm eggs.

Late instar beet armyworm larva.

Adult beet armyworm moth

Beet armyworm larvae can vary from light green to nearly black. They have four pairs of abdominal prolegs and a dark head capsule. Many fine, white wavy lines run along the back, and a broader stripe occurs along each side. There is usually a distinctive dark spot on each side, just above the second pair of true legs. Females lay eggs in masses, covering them with hairs and scales. Eggs hatch in a few days and larvae feed for about three weeks, sometimes spinning slight webs over the foliage.

Because beet armyworms do not appear to overwinter in Kansas, they have not been a problem until late summer or early fall. These foliage feeders usually are not a serious problem on established stands unless there are enough to severely defoliate plants. More severe problems occur in recently planted fields where seedlings may be lost when larvae clip them off near the soil surface. Economic infestations are similar to army cutworms – possibly four to five per square foot on established stands and one to two per square foot on seeding stands. Late fall infestations often disappear soon after the first hard freeze, but this is often difficult to predict. Beet armyworms are difficult to control, and options are limited.

Please refer to the most recent version of the Alfalfa Insect Management Guide for specific control alternatives.

Page last updated 10/29/2013 by J.P. Michaud.